The present invention relates generally to sensor domes, for example, antenna radomes. More specifically, the present invention relates to methods and systems for preventing ice from forming on antenna radomes.
Antenna radomes are provided in hostile environments as physical protection for antennas which transmit electromagnetic waves. Naturally, a primary concern in designing these radomes is that they do not adversely effect the transmitted or received electromagnetic waves and thereby reduce the effectiveness of the transmitting or receiving device (e.g., a radar). Radomes can adversely impact these transmissions in at least two ways. First, radomes can reduce the overall energy output of the transmitted waves by attenuating the waves as they pass through the radome. Second, radomes can distort or shift the phase of the waves so that the desired electromagnetic transmissions do not occur and, in the case of radar, returning electromagnetic waves are inaccurate.
Unfortunately, these problems lead to many design compromises. For example, continuous metal layers cannot be used to form the radomes since such materials would attenuate the electromagnetic waves to an unacceptable degree. Thus, various types of dielectric material are typically used to fabricate radome walls despite their generally inferior strength characteristics compared to metals.
Further complicating this situation is the problem of anti-icing. In many applications, radomes and antennas are disposed in environments where ice can form on the radome. For example, radomes located on airplanes or helicopters are highly susceptible to icing. Ice build-up on the outside surface of a radome compounds both of the above-described problems of attenuation and distortion of the transmitted electromagnetic waves. Not surprisingly, radome designers have been experimenting with methods and devices for preventing ice formation on radomes for some time.
One proposed anti-icing solution is to heat the air either in the interior of the radome or in ducts which are located within the radome walls. Heating the interior of the radome has been found to be ineffective in some situations because the radome's dielectric walls act as insulators and ice still forms depending on variables such as the environmental conditions, thickness of the radome walls, and amount of heat generated.
The solution of providing air ducts into the radome walls suffers from many drawbacks when actually implemented. For example, the resulting radome walls are bulky, complex to manufacture and lack structural integrity. Further, the asymmetrical nature of such radome walls tends to cause distortion of the outgoing electromagnetic waves.
Another solution is to incorporate resistive heating elements into the radome walls and pass current through the heating elements to heat the radome walls in a manner analogous to rear-window defrosters in automobiles. This solution is problematic, however, in that the heating elements also distort and/or attenuate the electromagnetic waves.
U.S. Pat. No. 4,999,639 to Frazita et al., discloses a radome having heating elements that are embedded or printed in the dielectric layers composing the radome walls. The heating elements are configured to provide impedance matching for the dielectric radome walls relative to the ambient environment. In this way, attenuation of the electromagnetic waves is allegedly reduced below the attenuation level that occurs from transmitting through the dielectric material alone. Moreover, the heating elements are spaced a distance of at most one-half of the operating wavelength of the antenna to minimize distortion.
However, the radome disclosed in the Frazita patent suffers from the drawback that it only prevents distortion or attenuation for transmitted electromagnetic fields having polarizations that are not parallel to the conductors embedded in the radome. Thus, this solution does not overcome anti-icing problems for radomes having antennas which transmit electromagnetic waves of varying polarizations.